The present invention relates to electrical measurement equipment and, in particular, to a range-changing circuit.
It is common in electrical measurement devices to have range selecting circuits on order to provide the desired range of measurements.
The simplest form is simply to use a selector switch to provide the desired range. More complex automatic range-changing circuit may use relays controlled by the value measured by the measurement device to select a range that puts the measured value within a desirable range. This process becomes more complicated as greater ranges, accuracies and speed are desired. Heretofore, these factors have created the need for more circuitry and complexity to produce the desired levels and performance.
Referring to FIG. 1, an exemplary prior art measurement device 1 is a source measure unit (SMU) in voltage control mode (voltage sourced, current measured). An error amp A1 controls QOUT. A series of current sensing elements and switches (except for the lowest range) are connected in parallel and are in series with the load resistor to provide range-changing. A differential amplifier senses across RLOAD and provides voltage feedback VFB. Feedback resistors R compare the VFB to the VDAC and present the corresponding error voltage to A1.
Switches S1 to SN may be electromechanical relays or solid-state switches with their attendant bootstrapping components to eliminate switch leakage. All the switches are controlled by a microprocessor in response to the voltage measured on a current sensing element selected by the microprocessor. The microprocessor thus choosing the desired range.
The switches are in series with the load; therefore an instantaneous change in RSENSE (by turning on a switch) will present a transient to the output. In the case of solid-state switches a “ramping” circuit is generally used to “fade” in the new element in parallel with the old, allowing the gain bandwidth of the loop to minimize the glitch. Because of the wide dynamic range of resistors, timing the ramps can be problematic and usually results in a tradeoff, where the higher current ranges are switched slower than they need to be, while the lower current ranges may have a larger transient. These glitches and transients are usually present at a time after the output has settled and are in response to measurements made by the A/D converter. If a different range is needed the corresponding process of selecting a range may make a transient that may take 100's of ms to settle at low currents. In many cases, noise may cause “hunting” in which a never-ending series of glitches is present.